The present invention relates to membranes employed, for example, in the separation of gases. In particular, the present invention relates to thermally stable membranes capable of withstanding high temperatures so as to be useful in the separation of hot industrial gases. 2. Prior Art
The use of membranes to separate gases from liquids, gases from other gases, and gases from solids is well known in the art. Typically, these membranes comprise a polymeric material such as polyethylene, polypropylene, polyvinylchloride, polytetrafluoroethylene, polyacrylonitrile, and in general polyesters and polyolefins, to name a few. In general, polymeric membranes are formed by preparing a solution of the polymer material, including any carrier solvents, hardeners, softeners, fillers, or the like; spreading the solution into a thin film by any well-known technique such as spraying, substrate dipping, or pouring a thin coating; and permitting the thin film to dry by means of ambient or heated air, for example. Polymeric membranes are employed for the purposes set forth above at ambient or room temperatures of about 25.degree. C. Occasionally, polymeric membranes are employed up to temperatures of about 100.degree. C. Most polymeric membranes decompose above 150.degree. C., thus the useful temperature range of polymeric membranes is relatively narrow.
U.S. Pat. 4,428,776 issued to Li is exemplary of polymeric membranes. The Li patent discloses a cellulosic semi-permeable membrane containing silicon compounds. The membrane is useful for the separation of various gaseous mixtures into their constituent parts, such as separating raw natural gas into enriched fractions of carbon dioxide and methane. The semi-permeable membrane is prepared from at least one cellulosic polymer and at least one silicon compound such as substituted silanes or siloxanes. In one embodiment, the unmodified cellulosic polymers are dissolved in a suitable solvent to form about 2 to about 20 weight percent solution. The resulting mixture is then poured over a clean glass plate and spread evenly to a uniform thickness with the aid of some instrument, typically a doctor's blade. The membrane is then air dryed, removed from the glass plate and further dried in air under ambient conditions for a suitable period of time, generally in excess of 24 hours. Additionally, the membranes are manufactured in structures other than films, such as hollow fibers, for example. Although the membranes are fabricated at any desirable thickness, membranes having a thickness less than about 25 mils tend to be most useful for the purposes described previously. These membranes are employed in the same manner as known membranes, i.e., a gaseous mixture to be separated is contacted with one side of the membrane in such a manner that one or more constituent parts of the gaseous mixture selectively pass through the membrane while the remaining constituent parts are rejected or prevented from passing through the membrane.
Recently, a molecular sieve carbon membrane, that contains no pores greater than those of molecular dimensions, was produced by pyrolysis of carbon containing compounds. The molecular sieve carbon membrane displayed gas permeabilities and selectivities that was considerably greater than most of the presently known polymer membranes. The pyrolysis was conducted at a temperature of 800.degree. C. to 950.degree. C. in the presence of substantially inert gas. Some oxidizing gas was employed in order to widen, by gradual burnoff, the pore size to achieve a specific permeability and selectivity. Pyrolysis of the membrane achieved ultramicroporosity, and it is theorized that the porosity was a result of small gaseous molecules channeling their way out of the solid matrix of the membrane during the pyrolysis. Thus the micro-pore structure was widened by oxidation burnoff to cause channeling of small gaseous molecules, or closed by high temperature centering which served to shrink the membrane. Molecular sieve carbon membranes were disclosed in the Journal Of Separation Science And Technology in 1983 by Dr. Abraham Sofer and Jacob E. Koresh.
The molecular sieve carbon membranes were employed to separate such gases as helium, oxygen, nitrogen, sulfur hexafluoride, and, to a lesser extent, carbon dioxide. However, these membranes, like the polymeric membranes, cannot withstand high temperature oxidizing gases and thus cannot be employed in the separation of hot industrial oxidizing gases because the molecular sieve carbon membranes disintegrate, particularly in an oxygen atmosphere, at temperatures above about 250.degree. C. to 300.degree. C.
Conventionally, high temperature separation of oxidizing gases from liquids or solids had been accomplished with porous ceramic, sintered metal or boronated glass in which the boron has been leached therefrom in order to produce a porous structure. While these materials were useful in separating gases from liquids and gases from solids, their use in separating gases from other gases was not well received due to the fact that the porosity of the above separation materials was overly large to the extent that very few gases would be retained by the separation materials. Moreover, when the separation materials were employed to separate gases from other gases, obtaining a uniform pore size in, for example, the leached boronated glass, was difficult and unsuccessful to the degree necessary to obtain adequate separation.
In summary, separation of hot industrial gases from liquids or solids could be accomplished by porous ceramic, for example. However, the materials capable of withstanding high temperature were not capable of separating gases from other gases. Polymeric membranes could be employed for separating gases from other gases, but these membranes were incapable of withstanding high temperatures.